cross-meson.mk: Fix build issue - conflict commits #4356 #4349

[th0ma7]

Motivation: Solves building using meson environment. Fix a merge conflicting changes between #4356 and #4349
Linked issues: #4379

Checklist

• Build rule all-supported completed successfully
• Package upgrade completed successfully
• New installation of package completed successfully

[hgy59]

@th0ma7 this was not a merge conflict. It was my intention to replace warning by error, after I successfully tested it.
But with #4396 I realize now, that it did not work when not using the spksrc-docker-image as build environment (for what reason ever...).

[th0ma7]

@hgy59 actually it's a little funnier than that: https://github.com/SynoCommunity/spksrc/commits/master/mk/spksrc.cross-meson-env.mk

1. fix meson build #4348 you added error
2. update meson build #4349 you migrated error to warning
3. fix dependency targets #4356 you re-inserted back error with the DEPENDENCY_WALK (perhaps using non-rebased code?)

[hgy59]

@th0ma7 exactly what I said
I migrated error to warning as errors where thrown with dependency-list target
After I was sure that dependency-list was fixed with DEPENDENCY_WALK I reactivated with error instead of warning.

Finally I am still confused why it was not fixed with DEPENDENCY_WALK as in #4396 the error was correctly thrown as meson was not installed.